For this dress, I finally took the time to make a muslin and am glad that I did. I ended up cutting one size smaller than the pattern called for and still had a fair amount of fitting at shoulder and under the bust. I took up an extra 1 1/4" from the front shoulder and an extra 5/8" in back. Then I cut down the back of the neckline 5/8" to account for these changes (to keep the neckline from ending up too far up the back of my neck). I also had to reshape the neck facing to fit the new neckline shape. I enlarged the front darts under bust and took up extra material under the arms as well. I also needed to take up the waist seam at the front to correct an unattractive dip.
After the fit changes, I also lengthened the sleeve to the elbow, omitted the sash and giant bow, added a 4" hem, bound the seams, sewed down skirt pleats and omitted pockets to reduce bulk, and added thread chains for the purchased belt and bra/slip straps.
Here is my muslin. Please forgive the Pepto Bismol print - it was $1.50/yd at Walmart. You can see in the first two pictures that the waist dips a little in the front, creating an unattractive barrel look at the midsection. I took up the waist seam a little between the front darts to correct this and create a straighter waist seam. You can also see the excess material at neck and the bulkiness below the belt. Again, this I corrected by removing the side-seam pockets and sewing down the pleats.
Here is the in-progress dress, showing my weird (but effective) front bodice darts, the skirt pleats being sewn down and the arduous process of easing the fullness out of 4" of hem on this full skirt... Next time I will listen closer when my Mama Leah says "I don't believe I'd put a 4" hem on that full skirt..."
And here is the final dress, as modeled by my dress form Gladys.